The invention relates to a circuit arrangement comprising at least two high-voltage power switches connected in series and having control means for repeatedly cutting off the switches. The control means are connected to a signal source for producing the control signal for the switches.
In such a circuit arrangement, which is generally known, a high voltage, which one switch alone is not capable of withstanding, is distributed over two or more switches. In this case the switches are controllable elements, for example, switching transistors, and it is necessary for these elements to be substantially simultaneously conducting and cut off. Switching on these elements is not very difficult because generally the switch-on time is very short. On the other hand, the switch-off time is comparatively long because the charge carriers stored during the preceding conduction time cannot be removed immediately, and the switch-off times of the various switches can be different. The differences, especially at high switching frequencies, are not negligible. Therefore, there is a risk that one switch may be still conducting while the other switch is already cut off, which, depending upon the circuit used, leads either to a non-uniform voltage distribution or to a current peak, as a result of which one or more of the switches can be damaged.
U.S. Pat. No. 3,631,314 discloses a circuit arrangement comprising one high-voltage power switch, which is switched off in a satisfactory manner in that its control lead comprises a delay element. This element consists of an inductor through which the control current for the switch, which is a power transistor, flows with a reduced rate of variation. Due to this current, the charge carriers leak away until the transistor comes out of saturation and is then cut off very rapidly, but a considerable time after the occurrence of the cut-off signal. When such switches are connected in a series arrangement to a high voltage, it would be obvious to provide the base lead of each switch with an adjustable inductor, the separate inductors being adjusted so that the transistors become non-conducting simultaneously. However, this process is time-consuming and moreover has to be repeated as the storage times of the charge carriers vary with time.